The present invention relates to a class of compounds to be used for the preparation of linear fluorinated vinylethers containing one fluorosulphonic group, and the preparation process thereof.
It is well known that fluorosulphonic vinylethers form a class of monomers useful for obtaining polymers containing xe2x80x94SO2F groups, which are used in electrochemical applications as membranes for chloro-soda cells, fuel cells or as acid catalysts in organic synthesis.
Compounds usable for the preparation of fluorosulphonic vinylethers are known in the prior art.
U.S. Pat. No. 4,358,412 describes the synthesis of the fluorosulphonic vinylether CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F, wherein in the first step the compound
FOCxe2x80x94CF(CF2Cl)xe2x80x94Oxe2x80x94(CF2)2SO2Fxe2x80x83xe2x80x83(I) 
is obtained by reacting the acylfluoride FOCxe2x80x94CF2xe2x80x94SO2F with the perfluoroallylchloride epoxide. In the second step the alkaline pyrolysis is carried out with sodium carbonate obtaining the fluorosulphonic vinylether. The synthesis of the formula (I) compound has the drawback to use the perfluoroallychloride epoxide which is very expensive and not easily available.
U.S. Pat. Nos. 4,962,282 and 4,801,409 describe the synthesis in gaseous phase of the hypofluorite FSO2CF2CF2OF starting, respectively, from the tetrafluoroethylene xcex2 sultone having the formula: 
or from the acylfluoride FSO2CF2COF.
The sulphonic hypofluorite FSO2CF2CF2OF can then be added, according to the prior art, to 1,2-dichloro-1,2-difluoroethylene, and then by dechlorination the monomer CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F is obtained. The sulphonic hypofluorite used in said process has the drawback to be a strongly oxidizing agent which requires particular precautions.
The need was felt to have available a class of compounds usable in the synthesis of the fluorosulphonic vinylether CF2- xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F, obtainable from easily available reactants, using the usual laboratory techniques.
A class of compounds which solves the above technical problem has been found.
An object of the present invention is a class of compounds, usable for the preparation of the fluorosulphonic vinylether CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F, formed by alkyl esters of the 2-(2-fluorosulphonyl)perfluoroethylenoxy-3-halogen-propionic acid, having the following formula (II):
CH2Yxe2x80x94CH(COOR1)xe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2Fxe2x80x83xe2x80x83(II) 
wherein:
Y=Cl, Br;
R1=C1-C4 linear or branched alkyl.
Preferably in formula (II) Y=Cl and R1=methyl.
The invention compounds are obtainable by the following process:
a) synthesis of the alcoholate of formula MOCF2CF2xe2x80x94SO2F, wherein M=K, Cs, Ag, by reaction between MF and TFE xcex2-sultone 
xe2x80x83or by reaction between MF and the acylfluoride FOCxe2x80x94CF2xe2x80x94SO2F;
b) reaction between a compound of formula CH2Yxe2x80x94CHXxe2x80x94COOR1 (III), wherein:
Y and R1 are as above;
X=Cl, Br; Y and X being equal or different, with the proviso that when Y=Br X=Br;
xe2x80x83with the alcoholate obtained in step a) according to the following scheme: 
The step a) is carried out in an aprotic dipolar solvent, for example diglyme (diethylen glycol dimethylether), acetonitrile, at temperatures in the range 20xc2x0 C.-50xc2x0 C.; the xcex2-sultone can be synthesized as described by England et Al. J. Am. Chem. Soc. 82 6181 1960.
The step b) is carried out at temperatures in the range 20xc2x0 C.-80xc2x0 C., by directly adding the alkyl halide RX to the reaction mixture obtained at the end of step a); at the end of the reaction it is distilled at reduced pressure and subsequently the distillate is extracted with water, the organic phase is separated and dried, recovering the product by solvent evaporation.
The invention results are surprising and unexpected since the reaction in step b) takes place selectively on the secondary carbon atom, bound to X in the formula (III) compound and it does not involve the primary carbon, bound to Y or that of carbonyl of the COOR1 group. This is surprising since one could expect that the reaction led to a product mixture, forming in low yields the final product. Surprisingly it has been found that said secondary reactions substantially do not take place and that the formula (II) compound is obtained in high yields. Furthermore the formula (II) compound is easily recovered from the reaction mixture.
The RX compounds are easily accessible and can be obtained by halogenation of acrylic esters.
As said, the invention compounds are usable for the synthesis of the fluorosulphonic vinylether of formula: CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F.
For example, the synthesis comprising the following steps can be used:
1) fluorination of the formula (II) compound to give the acylfluoride CF2YCF(COF)OCF2CF2xe2x80x94SO2F;
2) saponification of the acylfluoride to the corresponding carboxylate CF2YCF(COOxe2x88x92Me+)OCF2CF2xe2x80x94SO2F, wherein Me+ is the alkaline metal cation;
3) thermal decomposition of the carboxylate obtained in 2) and formation of the fluorosulphonic vinylether.
The step 1) fluorination is carried out either by electrochemical fluorination or by direct fluorination. The electrochemical fluorination is described in U.S. Pat. No. 2,713,593 and in patent application WO 98/50603. The direct fluorination is described in U.S. Pat. No. 5,488,142.
For example a tubular reactor, as described in U.S. Pat. No. 5,488,142, can be used for the direct fluorination, by adding the formula (II) compound to a circulating liquid flow of an inert solvent, for example perfluorohexane, to which elemental fluorine diluted with nitrogen is continuously fed at the temperature of 20xc2x0 C.
The acylfluoride saponification in step 2) is carried out by treatment with alkales at room temperature (20xc2x0-25xc2x0 C.).
Step 3) of the thermal decomposition is carried out by heating the compound isolated at the end of step 2) at a temperature in the range 50xc2x0 C.-220xc2x0 C., optionally dispersed in a solvent having a boiling point higher than that of the reaction product, inert under the reaction conditions, by distilling the formed fluorosulphonic vinylether.
The following Examples illustrate the invention and do not limit the scope thereof.